(1) Field of the Invention
This invention relates to a magnetic disk device and a magnetic disk used in such a disk device, and more particularly to a magnetic disk device of the so-called external synchronization system using a head assembly of the recording and reproducing heads separation type.
(2) Description of the Related Art
In magnetic disk devices, as means for obtaining a clock signal required in recording or reproduction of data, there are known so called a self-synchronization system of writing so called servo information onto a magnetic disk at the same time in recording data thereonto to reproduce the servo information to generate a clock signal on the basis of the reproduced servo information, or to reproduce the recorded data to extract the clock component from the reproduced data to generate a clock signal; and so called an external synchronization system (sample servo system) in which clock patterns are formed in advance in a discrete manner on a magnetic disk to reproduce any clock pattern to generate a clock signal on the basis of a signal of the reproduced clock pattern.
In actual terms, a magnetic disk device of the external synchronization system comprises, as shown in FIG. 1, a magnetic head 111 commonly used for recording and reproduction, a selector (changeover) switch 112 for switching the magnetic head 111 depending upon any selected one of the recording mode and the reproducing mode, a reproducing amplifier 113 for amplifying a reproduced signal delivered from the magnetic head 111 through the selector switch 112, a clock signal generator 114 (hereinafter simply referred to as a clock generator 114) for generating a clock signal on the basis of a reproduced signal corresponding to a clock pattern of the magnetic disk 101 amplified by the reproducing amplifier 113, a data demodulator 115 for reproducing data from the reproduced signal from the reproducing amplified 113 by using the clock signal from the clock generator 114, a timing generator 116 for counting the number of pulses of the clock signal from the clock generator 114 to control the clock generator 114, and to output a switching signal for controlling the selector switch 112, a recording data generator 117 for converting inputted data (hereinafter referred to as source data) to data suitable for recording (hereinafter referred to as recording data), and a recording amplifier 118 for delivering a current based on the recording data from the recording data generator 117 to the magnetic head 111 through the selector switch 112.
On the other hand, on the magnetic disk 101 used in this magnetic disk device, as shown in FIGS. 2(a) and 2(b), between data segments 102 serving as an area for recording data on recording tracks concentrically provided, radially continuous clock patterns 103 for clock generation (i.e., each clock pattern 103 for clock generation is radially continuous) are formed in advance by partially removing the magnetic layer which is the constituent of the magnetic disk, e.g., by using technique such as etching, etc. These clock patterns 103 are subjected to direct current magnetization (hereinafter simply referred to as d.c. magnetized) in one direction (indicated by an arrow), and are provided at about several hundreds.about.thousand (1000) portions per one circumference for the purpose of generating a high accuracy clock signal.
The magnetic head 111 reproduces a signal corresponding to data recorded on the data segments 102, and reproduces a signal corresponding to the clock pattern 103 to deliver these reproduced signals to the clock generator 114 and the data demodulator 115 through the selector switch 112 and the reproducing amplifier 113.
The clock generator 114 is comprised of, e.g., so called a PLL (Phase Locked Loop), etc., and serves to generate a clock signal on the basis of the reproduced signal corresponding to the clock pattern 103.
Namely, when clock pattern 103 d.c. magnetized in one direction is reproduced as shown in FIG. 3a, a reproduced signal having isolated waveforms at the forward and backward edges of each clock pattern 103 is obtained. The timing generator 116 counts the number of pulses of a clock signal delivered from the clock generator 114 to predict an occurrence (appearance) period of the reproduced signal corresponding to the clock pattern 103 on the basis of the past record (history) to generate a clock gate signal caused to be at high level (hereinafter referred to as H level) for this time period (occurrence period of the reproduced signal corresponding to the pattern 103) as shown in FIG. 3c to deliver this clock gate signal to the clock generator 114. The clock generator 114 considers an isolated waveform appearing within a time period for which the clock gate signal is at H level to be a normal clock pattern to update the phase of the PLL so that the rising of each clock signal (pulse) becomes synchronous with the peak of the isolated waveform corresponding to the forward edge, thus to generate a clock signal synchronous in phase with the clock pattern 103.
When the operation of the device is in the reproducing mode, the data demodulator 115 discriminates or demodulates a reproduced signal, e.g., by using a clock signal generated by the clock generator 114 to thereby reproduce data.
On the other hand, in the recording mode, by controlling the switching operation of the selector switch 112 by using a switching signal delivered from the timing generator 116, recording of data is carried out.
Namely, the timing generator 116 counts the number of pulses of a clock signal to thereby generate a switching signal which is caused to be at L level for a time period during which the magnetic head 111 scans the data segment 102 and is caused to be at H level for a time period during which the magnetic head 111 scans the clock pattern 103 to control the switching operation of the selector switch 112 by this switching signal. As a result, the magnetic head 111 is connected to the recording amplifier 118 when the switching signal is at L level and is connected to the reproducing amplifier 113 when it is at H level.
The recording data generator 117 converts source data to recording data synchronous with a clock signal generated at the clock generator 114 by a predetermined modulation suitable for recording. The recording amplifier 118 amplifies this recording data to deliver a current based on the recording data to the magnetic head 111 through the selector switch 112.
In actual terms, as shown in the above-mentioned FIG. 3b, for a time period during which the recording head 111 scans the clock pattern 103, the switching signal is caused to be at H level. Thus, the magnetic head 111 is connected to the reproducing amplifier 113. As described above, reproduction of the clock pattern 103 is carried out. Thus, a clock signal is generated. When a time period during which the magnetic head 111 scans the data segments 102 is initiated, the switching signal shifts to L level. Thus, the magnetic head 111 is connected to the recording amplifier 118. As a result, recording of data is carried out. Reproduction of clock patterns 103 and data recording are interchangeably carried out, whereby recording of data synchronous with the clock signal generated at the clock generator 114 is carried out as shown in FIG. 3f. Accordingly, in the reproducing mode, by discriminating a reproduced signal at the rising time of each clock signal (pulse) (hereinafter referred to as a data existing point phase) generated at the clock generator 114, the reproduced signal is referenced at the existing position in a running direction of the magnetic head 111 of data recorded on the data segment 102. Thus, data reproduction free from error can be carried out.
Meanwhile, so called a magneto-resistance effect type head (hereinafter referred to as a MR head) has an excellent characteristic, e.g., excellent sensitivity, S/N (Signal to Noise ratio), and spatial resolution. When this MR head is used for attaining high density recording, the above-described magnetic head 111 is, as shown in FIG. 4, composed of a reproducing head 111a comprised of MR head and a recording head 111b comprised of an ordinary magnetic head. Namely, independent heads for recording and reproduction (so called a head assembly of the recording and reproducing heads separation type) are used. In this case, however, problems as described below would take place.
As described above, in the magnetic disk device of the external synchronization system, the phase of the data existing point is determined on the basis of a reproduced signal corresponding to the clock pattern 103 obtained by the reproducing head 111a. When the operation is in the reproducing mode, the position of the reproducing head 111a at the data existing point phase is caused to be the data existing point position on the data segment 102. Accordingly, in recording data, it is necessary to carry out recording so that the data existing point phase and the data existing point position on the data segment 102 when viewed from the reproducing head 111a are in correspondence with each other at all times. However, in the case where the recording head 111b and the reproducing head 111a are separate, the data existing point phase and the data existing point, position on the data segment 102 when viewed from the recording head 111b are not in correspondence with each other in general by the physical position relationship between both heads. In more practical sense, as shown in the above-mentioned FIG. 4, the recording head 111b is attached relative to, i.e., away from the reproducing head 111a by a length L in a backward direction. Accordingly, in carrying out recording, recording operation must be conducted after the reproducing head 111a at the data existing point phase is moved to the position of the recording head 111b with rotation of the disk.
A time T1 (phase shift) required for the above-mentioned movement is determined by a length L between the reproducing head 111a and the recording head 111b and a relative velocity v between the head 111 and the magnetic disk 101. Namely, this movement time T1 is expressed as T1=L/v. In a magnetic disk device adapted to rotationally drive magnetic disk 101 at so called a CAV (Constant Angular Velocity), since the relative velocity v between the head 111 and the magnetic disk 101 varies depending upon the position in a radial direction where the head 111 exists, the movement time T1 will be also dependent upon the position in the disk radial direction of the head 111. A magnetic disk device adapted to record data by taking such phase shift into consideration is disclosed by this applicant in the Japanese Patent Application No. 276165/1992 (not yet laid open as of Nov. 30, 1993).
Further, in the magnetic disk device, a recording current caused to flow in the recording head 111b is inverted to produce reversal of magnetization on the data segment 102 to thereby record data. However, in the case where the recording density is high, a phenomenon such that the position of reversal of magnetization remaining on the data segment 102 is shifted relative to the position where the recording current is inverted (which phenomenon will be hereinafter referred to as a non-linear bit shift) occurs as shown in FIG. 5. It is known that the direction and the magnitude of shift in the non-linear bit shift is dependent upon the pattern of data to be recorded. In order to allow reversal of magnetization to remain at a normal position on the data segment 102, in carrying out recording, the inverting position of the recording current must be moved in advance in a direction opposite to the direction where shift takes place. The above-mentioned application (Japanese Patent Application No. 276165/1992) neither describes or suggests such non-linear bit shift.
As stated above, in the magnetic disk device of the external synchronization system using head (assembly) 111 of the recording/reproducing heads separation type, any time difference occurs between a data existing point phase generated on the basis of clock pattern 103 and a time at which the recording current should be actually inverted, viz., a phase shift of data recorded onto the magnetic disk 101 resulting from distance L in a running direction between the reproducing head 111a and the recording head 111b and a positional shift of reversal of magnetization resulting from the pattern of recording data take place. In addition, since those shifts vary depending upon the position in a disk radial direction of the head 111, or the pattern of data to be recorded, it is impossible to record data at a correct position. As the technology for allowing the influence of the non-linear bit shift to be small, U.S. Pat. No. 4,964,107 (International Business Machines Corporation, Oct. 16, 1990) is known. Namely, a circuit for a partial-response, maximum likelihood (PRML) magnetic recording channel stretches and shrinks pulses in particular write-data sequences. The circuit maintains precise tracking in the delays among multiple signals by sending them through the same number of identical circuits on the same chip. An external digital code varies the amount of delay in a clock signal so as to stretch and shrink the data pulses by different amounts. This patent describes the non-linear bit shift, but neither describes or suggests the problem of the phase shift of data recorded onto a magnetic disk resulting from the distance in a running direction between the reproducing head and the recording head.
Accordingly, a magnetic disk device capable of solving both problems of the phase shift and the non-linear bit shift is required.